Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.
Concrete is marvelous stuff but in time it will deteriorate. When it does you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jack hammer and bags of one of the Sakrete concretes.
Since this discussion is on the best way to bond concrete we will assume that your slab is good.
There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also keep your thumb out of the way. If the spots are too large or too deep for this to be practical you may need a sealer to cover the stains before patching.
There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.
Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.
The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it just ain’t so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When you use a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.
If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day the existing concrete surface will be hot enough to suck the water our of the repair material. In addition some concretes are quite porous and will also rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result.
There are some substances that concrete simply will not bond to. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.
Once the job is complete you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.
© 2012 Sakrete
USER COMMENTS
No comments for all the above. I do have a question though. What's your opinion about restoring residential garage floor with the flowing material? I have to mention salt-chipping surface from road salt. Thank you.
- Stavros
Friday, February 11, 2011 at 8:00 PM
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Stavros, You can repair your garage floor with the Flow Coat Concrete Resurfacer. This material can be placed from 1/2" down to a featheredge in one lift. You will need to make sure that the surface of the the floor is clean and free of any loose material, dust, debris, paints, and sealers. If you need anything else please feel free to call us at 1-866-SAKRETE and we will be glad to assist you.
- Lee
Tuesday, February 15, 2011 at 9:01 AM
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Where can I get a bag of scratch coat?
- Brian
Saturday, March 26, 2011 at 11:46 AM
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As stated above... "This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop." The scratch coat is not a special bagged product.
- slw
Monday, March 28, 2011 at 11:38 AM
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My front porch has edge worn off severely. I mean edge is worn off back 3 inches or more in some places. Lowes guy told me to get Bonding agent and Sakrete sand mix. So here is my plan: I have chipped off and swept off all loose material. I will lightly spray off dust with garden hose. Apply bonding agent. Apply scratch coat. Quickly place a board up against porch edge (bottom is still intact) and place blocks against to hold in place. Mix up and apply the repair material. Does this sound right?
- Fred
Tuesday, April 5, 2011 at 8:03 PM
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Fred, in order to recommend the correct Sakrete products we need to gather additional information on your project. Could you please contact our Technical Service Department at 1-866-SAKRETE. There are many variables with your application that we would like to discuss prior to recommending a product.
- Dean
Wednesday, April 6, 2011 at 12:14 PM
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My patio got a crack in it last year when the weather was extremely hot for my area. I don't want to tear up all the concrete. Can I just put down about an inch layer of new concrete over it. I was thinking using the crack resistant cement. The slab is about a 10x12. Would it be feasable to fix this way?
- Dan Williams
Wednesday, April 13, 2011 at 1:07 AM
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Dan, We would not recommend applying a 1" layer of Crack Resistant Concrete over the existing cracked slab. If you were to apply the material without addressing the crack then it could easily crack again. Please contact us at 1-866-SAKRETE to speak with one of our Technical Service Representatives for more information on correct repair materials.
- Dean
Thursday, April 21, 2011 at 11:15 AM
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Crack Resistant is a concrete mix and would'nt be used for anything under 2 inches...really I would'nt use for anything under 3 inch..
- Joe
Monday, April 25, 2011 at 3:31 PM
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My patio has pulled away from the house and dropped about about an inch. There is now a 6 inch gab between the house and patio. Is there any way to repair this without redoing the whole patio?
- Tom Waters
Tuesday, July 5, 2011 at 6:34 PM
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Tom, due to the fact that there is movement in the patio any repairs may only be temporary. It would be recommended to address the cause of movement prior to any repairs. It could be that there is not a proper drainage bed under the patio which has caused it to move. Any materials placed over the patio will not prevent further movement and cracking could occur.
- Dean
Wednesday, July 6, 2011 at 10:37 AM
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I have a concrete exterior stairs with a verticle side wall going down to my basement. The top 2 stairs but to the foundation and have gaps in the edges to the foundation allowing water to leak through when ground water rises, and the side wall has cracked away about .75" showing ruff stones now. I want to patch both areas and wa thinking about your top and bond product for this repair?
- chuck
Sunday, August 28, 2011 at 2:15 PM
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Chuck, for your application we would recommend using a polyurethane product like the Sakrete Non-Sag Sealant that has some flexibility. You can try using the Top'n Bond however it will require more maintenence than the Non-Sag Sealant since it is a rigid material and may crack again if theres any movement. The gaps will need to be filled in with backer rod prior to applying the Non-Sag Sealant and the depth should be no more than 1/2".
- Dean
Monday, August 29, 2011 at 10:50 AM
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Can I send you a photo of my concrete stairs and perhaps recommend a product to repair?
- Arlene
Monday, September 19, 2011 at 4:50 PM
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Absolutely, if you select "Share Your Project" under the Contact Us drop down box at the top of the page you are able to attach photos of your project. It will then be sent to the Sakrete technical service department where we will be able to assist you.
- Dean
Tuesday, September 20, 2011 at 10:42 AM
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What is teh difference between Top N Bond and Flo Coat? I need to resurface my patio.
- James
Wednesday, October 19, 2011 at 12:39 PM
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There are actually quite a few differences between the Top'n Bond and the Flo-Coat Resurfacer. The main difference you will notice is with the consistency and application method. Flo-Coat is applied in a flowable consistency with a squeegee while the Top'n Bond will have a thicker consistency and is applied with a trowel. If it is a large project you may want to use the Flo-Coat since it can be applied much easier and for smaller projects and for filling in cracks it may be best to use the Top'n Bond. There is also a difference in the compressive strengths. The Flo-Coat has a compressive strength of 4,500 psi after 28 days while the Top'n Bond will be 5,000 psi. Both products are designed for applications from 1/2" down to a feather edge.
- Dean
Thursday, October 20, 2011 at 11:50 AM
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I have built a storage area with a 8ft opening on an existing slab. When it rains the water flows in through the opening. Is there some kind of permanent barrier (two sided ramp)I can adhere to the concrete that can with stand weight and gentle abuse?
- David
Friday, January 20, 2012 at 12:42 PM
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David, are you trying to build a concrete ramp?
- Dean
Friday, January 20, 2012 at 2:14 PM
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Yes,I would like to build a ramp. Nothing to big.It would need to be sloped on both sides.
- David
Saturday, January 21, 2012 at 12:16 PM
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I have a sloping basement floor that I am levelling prior to tiling. It has an oval depression (to a floor drain) about 6'x10'. I've tried Sakrete sand mix, but it didn't bond (photo), so I removed it. I'm now thinking of using a Latex fortified thin set with a dry pack. The photo shows whats left of the old residual flooring adhesive (black). Should I use a liquid bonding coat as well? What products would you recommend for the thin set and the dry pack?
- Bill
Sunday, January 22, 2012 at 12:04 PM
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David, the product you will need to build a ramp will depend on the thickness. If you are wanting to slope it from 1/2" to a featheredge you can use the Top'n Bond. For a larger ramp 2" - 1/2" you can use the Fast Setting Cement Patcher. Make sure the concrete is properly prepared before applying the repair materials, no paints, oil, or anything that will prevent a bond. The Top'n Bond is already modified so no bonding agent is needed but with the Fast Setting Cement Patcher you can brush on some Bonder & Fortifier on the existing concrete to enhance the bond.
- Dean
Monday, January 23, 2012 at 9:11 AM
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Bill, it sounds like the old flooring adhesive prevented the Sand Mix from bonding properly. The adhesive must be mechanically removed in order to achieve good results. Once all of the adhesive is removed then you can apply the materials. The Sand Mix can be used as a dry pack. Unfortunately we don't manufacture thin set mortars but we do recommend using a ProSpec thin set product. If you have any questions about this please contact us at 1-866-SAKRETE.
- Dean
Monday, January 23, 2012 at 10:23 AM
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My project is leveling my basement from approximately 1-1/4" to a feather edge. We used 100 bags of top'n bond, we did not rough the surface and we used a bonding agent on the old floor. Obviously the entire 400 square feet did not bond and it shrunk. It was approximately 100 bags of cement. What's upsetting is that the directions on the bag say nothing about not using a bonding agent on the old floor. Directions on the sakrete flo-coat do have directions that say not to use a bonding agent. My new approach is this: - rough the surface with a concrete scarifier - wet the floor overnight but do not produce puddles - use 100 bags of sakrete flo-coat - force a scratch coat into old concrete before pouring new concrete - apply in 1/2" thickness in 24 hour intervals. Will my new approach work? Can I get a discount on my next 100 bags?
- Adam
Saturday, February 18, 2012 at 10:12 PM
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Adam, based on the information provided there are a few reasons for why the overlay possibly did not work properly. Surface preparation is very important, and going over a smooth surface will be difficult if it is not roughed up. The Top'n Bond is designed for applications from 1/2" down to a featheredge. Going much thicker than this in one application or adding more than the recommended water can cause shrinkage problems. Also any coatings or sealers will be bond breakers, preventing the Top'n Bond from adhering. Since it is in your basement if the slab has a high moisture content it can cause bonding issues as well. For this type of application it would be best to speak with one of our technical representatives to ensure that we have all the information needed to recommend the right products. You can contact us at 1-866-SAKRETE.
- Dean
Tuesday, February 21, 2012 at 5:15 PM
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when can I use Flo-coat on a new concrete walkway? how long does the new concrete needs to cure before flo-coat application. Thanks
- tom
Sunday, April 29, 2012 at 10:34 PM
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Tom, when using Sakrete Flo-Coat Resurfacer it is recommended to apply it to cured concrete which would be 28 days.
- Dean
Monday, April 30, 2012 at 4:16 PM
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I have an old poured basement from the 1930's and there is a high sand content and the inside walls are spalling all over the place. The base ment is dry and I do not have water problems it just looks really bad. What would you recommend for this type of repair?
- Jeff
Thursday, May 10, 2012 at 10:41 PM
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Jeff, back during that time frame ash was commonly used in concrete mixes which could have produced a lower strength concrete and for that reason I would recommend using the Type N Mortar Mix. The surface needs to be prepped really well before hand, make sure you remove any loose material and that it's clean with no paint, grease or coatings. Also, you should apply Sakrete Bonder & Fortifier on the wall to make sure you get a good bond. Keep in mind that this is a patch and if there are any structural issues they should be addressed by a professional.
- Dean
Friday, May 11, 2012 at 2:41 PM
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I looked on the net for definition of frame ash but all that came up was fly ash. Is it the same thing? Do you have an opinion on if it is all right to insulate these types of walls with foam board. Home is in Wisconsin.
- Jeff
Saturday, May 12, 2012 at 10:26 AM
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Jeff, sorry for the misunderstanding I just meant during that time frame it was used in concrete. As far as insulating the walls with foam board unfortunately we wouldn't have any recommendations on that.
- Dean
Monday, May 14, 2012 at 9:39 AM
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